Martin Weitzman: Recently the concentration of atmospheric carbon dioxide (CO2) reached an unprecedented level of 400 parts per million. What is the significance of this “milestone”? Does it portend catastrophic climate change? The short answer is no. The long answer is a more complicated and more nuanced maybe.

The modern era of carefully measuring and recording atmospheric CO2 began with the work of famed scientist Charles Keeling. In 1958, Keeling began to accurately monitor daily CO2 levels atop Mauna Loa, the highest mountain in Hawaii. Keeling chose this location because it was so remote from manmade sources that it would accurately track average “well mixed” CO2 levels throughout the world. Thanks to Keeling’s pioneering work we now have a continuous ongoing record of CO2 levels since 1958.

In 1958, Keeling recorded an atmospheric CO2 level of 315 ppm. Every year since then the Mauna Loa station has recorded ever-higher levels of CO2 than the year before. Atmospheric CO2 concentrations have grown relentlessly over the years until they just recently blew past the well-publicized milestone of 400 ppm.

The 400 ppm milestone is basically just a round number. To see why it might (or might not) be viewed as something unusual, or even threatening, we need to examine a longer record of CO2 levels over time.

There is a remarkable record of CO2 concentrations preserved in tiny bubbles in Antarctic ice cores going back 800,000 years. These measurements are less accurate than modern Keeling-style instrumental readings, but they are plenty accurate enough to see the big picture clearly. All throughout the past 800,000 years, which encompasses several ice ages and interglacial warming periods, CO2 levels fluctuated in a relatively narrow band between about 180 ppm (during the colder ice ages) to 280 ppm (during the warmer interglacial periods). For about the last 10,000 years we have been living in a warm interglacial period, with CO2 concentrations at about 280 ppm. Then, beginning with the industrial revolution about 1750, CO2 concentrations gradually moved up to Keeling’s accurately measured 1958 level of 315 ppm. Since then, as we have seen, CO2 concentrations have grown rapidly to the current 2013 level of 400 ppm.

So, the current CO2 concentration of 400 ppm is some 40 percent higher than anything that has been attained in the last 800,000 years. The glacial-interglacial cycles began some two and a half million years ago. Scientists estimate that a CO2 concentration of 400 ppm has not been attained for at least 3 million years. This rapid a change in CO2 concentrations has probably not occurred for tens of millions of years.

The point here is that we are undertaking a colossal planet-wide experiment of injecting CO2 into the atmosphere that goes extraordinarily further and faster than anything within the range of natural CO2 fluctuations for tens of millions of years. The result is a great deal of uncertainty about the possible outcomes of this experiment. The higher the concentrations of CO2, the further outside the range of normal fluctuations is the planet, and the more unsure are we about the consequences. [Continue reading…]